Electro Active Polymers (EAP) in electromechanical energy conversion applications require active excitation in each electrical or mechanical cycle. Generally, the material is charged at the beginning of the cycle, and discharged at the end of the cycle. It is this charge biasing that allows the electromechanical conversion process.
For typical applications having cycles with relatively small deformations (<50%), the energy required to bias the material is much larger than the net amount of energy that is converted. Given the high series resistance of present EAP devices, the energy lost in the charging and discharging phase easily exceeds the converted energy. Therefore, high conversion efficiencies can only be achieved by a highly efficient charging and discharging process.
An electromechanical energy conversion system using an EAP based device is for example disclosed in WO 2010/146457. Such an EAP based device can be considered as a variable capacitor of which the capacitance changes as a function of the amount of deformation exerted on a layer of EAP material.
In the already known EAP charging and discharging systems, a switching DC/DC or AC/DC Power Electronic Converter (PEC) performs the charging and discharging of a variable capacitor from/to a power source/sink. Typically, charging is performed by a step-up converter, such as a boost converter, while the discharging is accomplished by a step-down converter, such as a buck converter.
FIG. 1 shows schematically a part of a circuit of a prior art electromechanical energy conversion system with parallel a boost converter and a buck converter between a power source/sink and an EAP based device.
The circuit 1 of the electromechanical conversion system comprises a low-voltage (Ub<Uvc) power source LV and a variable capacitor 10 based on an elastically deformable body of an EAP material. The power source is arranged to act as power source during charging of the variable capacitor and as power sink during discharge of the variable capacitor.
The power source LV and the variable capacitor 10 are coupled to each other by a parallel arrangement of an step-up converter (boost converter) L1, S1, D1 and an step-down converter (buck converter) L2, S2, D2.
The step-up converter L1, S1, D1 comprises a boost inductor L1, a boost switching element S1 and a boost diode D1, wherein the boost inductor L1 and the boost diode D1 are arranged in series between the positive terminal of the power source LV and the positive electrode (plate) of the variable capacitor 10. The forward direction of the boost diode D1 is in the direction towards the positive terminal of the variable capacitor 10.
The negative terminal of the power source LV and the negative electrode of the variable capacitor 10 are directly coupled by a line 11. The boost switching element S1, typically a transistor, is arranged with one terminal of the switch connected between the boost inductor L1 and the boost diode d1 and the other terminal of the switch connected to the line 11.
The step-down converter L2, S2, D2 comprises a buck inductor L2, a buck switching element S2 and a buck diode D2, wherein the buck inductor L2 and the buck switching element S2 (typically a transistor) are arranged in series between the positive terminal of the power source LV and the positive electrode (plate) of the variable capacitor 10. The negative terminal of the power source LV and the negative electrode of the variable capacitor 10 are directly coupled by a second line 12. The buck diode D2 is arranged with one terminal connected between the buck switch S2 and the buck inductor L2 and the other terminal of the diode D2 connected to the second line 12.
Note that the highly resistive EAP device (the variable capacitor 10) is on the load side during charging and on the source side during discharging.
Inherent to this arrangement of the switching DC/DC step-up and step-down converters L1, S1, D1; L2, S2, D2 is that during operation either the load side current (step-up) or the source side current (step-down) is being interrupted by a high-frequency on/off switching by means of the switching element S1; S2. The resulting discontinuous current increases the losses in the series components significantly, because of a higher effective current value. As known to the skilled in the art, the power being converted in such switching DC/DC step-up and step-down converters is related to the average current, while the effective current is relevant for the losses.
It is an object of the present invention to provide an electromechanical energy conversion system and a method for such a system that overcomes the disadvantage from the prior art.